First off, postdoc salary levels at Caltech vary widely by department.
Regarding taxes, there are tax treaties between the US and a lot of other countries. For some of these countries, these treaties have special regulations for (postdoctoral) researchers staying for less than a certain time (e.g. 2 years): If you move to the US from such a country (not your home country, but that of your last residence), you remain tax resident of that country, so your postdoc salary remains taxable in that country, rather than the US. (Note that you still have to pay state taxes in California, but these are rather low.) I think the philosophy is that you do not give up your residence in said country, but only come for a "long visit". There are various rules whether you can choose where you tax your income or not, and what happens if you exceed the period of time for which you are subject to this tax exemption, which differ by country of origin, so essentially you have to check the relevant tax treaty. (Notably, with some tax treaties, if you exceed that time you have to tax all of your income in the US retroactively.)
As an example, here is an excerpt from the US-German tax treaty (2006 Amendment, Article XI):
Remuneration that a professor or teacher who is a resident of a Contracting State and
who is temporarily present in the other Contracting State for the primary purpose of
carrying out advanced study or research or for teaching at an accredited university or
other recognized educational institution, or an institution engaged in research for the
public benefit, receives for such work shall be taxable only in the first-mentioned
Contracting State for a period not exceeding two years from the date of his arrival.